Medical robotic systems such as those used in performing minimally invasive surgical procedures offer many benefits over traditional open surgery techniques, including less pain, shorter hospital stays, quicker return to normal activities, minimal scarring, reduced recovery time, and less injury to tissue. Consequently, demand for minimally invasive surgery using such medical robotic systems is strong and growing.
Examples of medical robotic systems include the daVinci® Surgical System and the daVinci® S™ Surgical System from Intuitive Surgical, Inc., of Sunnyvale, Calif. Each of these systems includes a surgeon's console, a patient-side cart, a high performance three-dimensional (“3-D”) vision system, and Intuitive Surgical's proprietary EndoWrist™ articulating instruments, which are modeled after the human wrist so that when added to the motions of manipulators holding the surgical instruments, they allow at least six degrees of freedom of motion, which is comparable to or even greater than the natural motions of open surgery.
The daVinci® surgeon's console has a high-resolution stereoscopic video display with two progressive scan cathode ray tubes (“CRTs”). The system offers higher fidelity than polarization, shutter eyeglass, or other techniques. Each eye views a separate CRT presenting the left or right eye perspective, through an objective lens and a series of mirrors. The surgeon sits comfortably and looks into this display throughout surgery, making it an ideal place for the surgeon to display and manipulate 3-D intraoperative imagery.
The patient-side cart typically includes three or more robotic arm assemblies with corresponding slave manipulators for holding and manipulating medical devices such as surgical instruments and image capturing devices for performing and/or viewing a medical procedure at a surgical site within a patient. To manipulate these medical devices, the surgeon's console also includes master input devices which may be selectively associated with the medical devices and their respective slave manipulators. Since the movements of the master input devices and their associated medical devices are scaled, this allows the surgeon to perform intricate medical procedures with greater ease than conventional open surgery. Further, it may even allow the surgeon to perform medical procedures that are not even feasible using conventional open surgery techniques.
To perform a minimally invasive surgical procedure on a patient, one or more incisions are first made in the patient and cannulae inserted therein to gain access to a surgical site within the patient. Setup arms supporting the slave manipulators are then positioned so as to allow the slave manipulators to attach to respective of the cannulae. Surgical instruments engaged on the slave manipulators are then inserted into the cannulae and properly positioned and oriented in order to perform the procedure. A surgeon may then manipulate master manipulators (or master input devices) which are coupled to the slave manipulators and their respective surgical instruments through one or more controllers to perform the surgical procedure.
The initial positioning and orientating of the surgical instruments at the surgical site is generally performed by an assistant who is positioned next to the patient and manually moves their respective slave manipulators so as to move the surgical instruments into their proper positions and orientations at the surgical site. Typically such positioning and orientating involves a two-step procedure in which, in a first part, each slave manipulator is attached to its assigned cannula and its surgical instrument engaged on it, and in a second part, the slave manipulator is manipulated so that its surgical instrument is properly positioned and oriented at the surgical site to perform its role in the minimally invasive surgical procedure. Although described as a two-part procedure for each slave manipulator, it is to be appreciated that the assistant may perform both parts concurrently or otherwise in an overlapping fashion, as well as sequentially so as to be performed one after the other.
To perform the first part of the initial positioning of a surgical instrument, the assistant depresses a first release button on a slave manipulator, which releases brakes holding setup joints of its corresponding setup arm in place so as to allow movement of the slave manipulator. The positioning of the slave manipulator is conventionally facilitated by the use of gravity-balanced or non-gravity loaded setup joints. After the slave manipulator is positioned so that it can be attached to its assigned cannula and is attached to it, the assistant may then stop depressing the first release button, which causes the brakes to hold their corresponding setup joints in place, thus locking the translational position of the cannula attached to the slave manipulator at this point. Additional details in performing this part of the procedure and the general construction of slave manipulators and their setup arms as pertaining to such procedure are provided in commonly owned U.S. Pat. No. 6,246,200 entitled “Manipulator Positioning Linkage for Robotic Surgery,” which is incorporated herein by this reference.
To perform the second part of the initial positioning of the surgical instrument, the assistant engages the surgical instrument onto the slave manipulator so that it is capable of inserting it into the cannula, pivoting the instrument around a pivot point located at the surgical port of the cannula, and driving an end effector at the distal end of the surgical instrument with degrees of freedom resembling wrist motion. It may be noted at this point that the surgical instrument tip is inside the cannula and thus, is no longer visible to the assistant since it is now shielded by the anatomy.
To proceed, the assistant depresses a second button on or proximate to its slave manipulator, which disengages active joints of the slave manipulator from being controlled by their associated master manipulator. This allows the assistant to freely move the slave manipulator to insert the surgical instrument into its cannula and pivot about it about the incision so as to point in the proper direction and at the proper distance into the incision. If an endoscope has been previously positioned in the patient to view the surgical site, typically this step is performed while the assistant views the image of the surgical site provided by the endoscope. After the surgical instrument is thus positioned at the surgical site, the assistant may then stop depressing the second button, which allows the surgeon to re-engage control of the active joints of the slave manipulator through the master manipulator so that the surgeon may perform the surgical procedure by manipulating the master manipulator. To facilitate the positioning tasks described above, the first and second buttons may be effective only while they are depressed (and thus work as momentary buttons) or they can remain effective after they are released until they are depressed again (and thus work as toggle buttons).
To facilitate easy movement of the slave manipulator during the second part of the positioning procedure, the slave manipulator may be mechanically gravity balanced using, for example, counter balance weights, so as to significantly reduce gravity effects and consequently, the force necessary for a person to physically move the slave manipulator. Additionally, the slave manipulator may be designed so as to have very high mechanical efficiency so that the friction does not change much with the force applied to the manipulator, have low friction forces or torque to overcome when moving the manipulator, and have low mass or inertia so that the manipulator may be accelerated with low force or torque.
Although a slave manipulator that does not have all of these mechanical characteristics may not be easy to manually position quickly and accurately, it may be advantageous to give away some of these mechanical characteristics in exchange for other benefits such as larger workspace, smaller footprint, or a lighter slave manipulator. In such a design, a control system that is capable of recovering the lighter feeling of the mechanically gravity balanced design is desirable.
One way of recovering the lighter feeling provided by counterweights while manually moving the slave manipulator is to use joint motor torques to exert assisting forces to effectively balance the gravitational weight of the slave manipulator. Using this approach, the joint motors of the slave manipulator are generally capable of applying torques greater than those necessary for normal control activities. Consequently, when the control of the slave manipulator is optimized for tracking purposes (i.e., to follow its corresponding master input device), for instance by the adoption of large control gains, the slave manipulator may be difficult to backdrive (i.e., cause its associated master input device to follow externally applied movement of the slave manipulator). Also, when the control of the slave manipulator is optimized for tracking purposes, it may tend to spring back quickly and overshoot its commanded position after it has been temporarily displaced by a large amount away from its commanded position by an externally applied force. Each of these characteristics tends to make the slave manipulator human unfriendly to the assistant during setup and to the patient during the performance of a medical procedure.